A heterodyne receiver, with either double down conversion or single down conversion, is widely used in a base station, a User Equipment (UE), or the like communication systems. At an Antenna Reference Point (ARP) of such a receiver, a received spectrum is densely occupied with both transmit and receive bands of various communication standards, such as Universal Mobile Telecommunications System (UMTS), Long-Term Evolution (LTE) or Advanced LTE (LTE-A). A band select filter attempts to allow only a spectrum, in which users of a particular standard are allowed to communicate to pass and attenuate all other standards and out-of-band interferences. The filtered signal is then amplified by a Low Noise Amplifier (LNA) to suppress contribution of noise from succeeding stages. The receive band is fairly wide with the desired channel embedded within interfering signals and its image. Therefore, the subsequent proceeding stages attempt to isolate the channel of interest while maintaining linearity and system performance.
FIGS. 1a and 1b respectively illustrate structures for a typical double down conversion heterodyne receiver and a typical single down conversion heterodyne receiver.
As show in FIG. 1a, for the double down conversion heterodyne receiver, the first mixing stage places an Intermediate Frequency (IF) at a higher frequency allowing maximal image suppression while the second mixing stage optimizes channel filtering. LOs for the first mixing stage and the second mixing stage are referred to as RF (Radio Frequency) LO and IF LO, respectively. Finally, an anti-aliasing filter with a sharp cutoff is required to reduce RF/IF harmonics. As shown in FIG. 1b, the single down conversion heterodyne receiver has only one stage IF and relative filtering.
The traditional receiver, such as those illustrated in FIG. 1a and FIG. 1b, only supports a single carrier, i.e., a single standard. It uses a narrow IF filter with high selectivity at a fixed frequency, thereby simplifying a design of the receiver. Characteristics of the IF filter are normally selected to match narrow channel requirements, such as channel bandwidth, filter skirt steepness, etc.
With the development of the mobile communications, a base station with multiple carriers will allow a module to be configured with multi-carriers.
For such a receiver, the whole available bandwidth is full band, and the physical bandwidth of an IF filter or an anti-alias filter must also be equal to or larger than the available bandwidth. But for most of the base stations, they maybe use portion of full available bandwidth, for example only one 20 MHz LTE carrier for previous full band (60 MHz) receiver, and only this used 20 MHz is the desired channel.
Moreover, there is a very common situation that an operator only has license of 20 MHz frequency band, but operates with full band (60 MHz) receiver hardware.
FIG. 2 shows relationship between UL band, receiver available bandwidth and the desired channel. Here, assume that the maximum received signal available bandwidth is 60 MHz, and a total design bandwidth of an IF filter and an anti-alias filter is also 60 MHz, as shown in the upper figure of FIG. 2.
In the current IF configuration, at a receiving end, there is not extra consideration for a signal frequency allocation when a bandwidth of the desired channel is less than the available bandwidth. A default configuration sets a signal frequency at the center of the IF filter.
As shown in the lower figure of FIG. 2, a received desired channel of 20 MHz is rather less than the available bandwidth of 60 MHz. Normally, there are two 20 MHz free spaces respectively around dual sides of the received signal channel of 20 MHz. For the current LO configuration, if any interference falls in the total 2*20 MHz in band of the IF filter bandwidth, the interference will not be attenuated by an analog part of the receiver (including the IF filter and the anti-alias filter).
Such a receiving carrier frequency allocation leads to obvious drawbacks that blocking interferences, which are at sides of the received desired channel of 20 MHz but still in the receiver available bandwidth, will arrive at an Analog-to-Digital Converter (ADC) of the receiver without any attenuation.
FIGS. 3a and 3b illustrate direct blocking impact and Third-order Intercept Point (IP3) blocking impact according to the prior art, respectively.
As shown in FIG. 3a, a high-level interference signal from UE B or an in-band interference source enters an available bandwidth of BS A directly. An analog gain of BS A from its antenna to its ADC must be reduced to avoid over-driving ADC, and then a noise figure of a receiver of BS A will go up. As a consequence, the receiver's sensitivity is degraded directly, and in a worst case, BS A can't talk to UE A when it is at the cell edge.
As shown in FIG. 3b, an IP3 product generated by BS A's transmitter leakage (i.e., TX leakage) at f3 and an interference at f2 from UE B or an in-band interference falls into BS A's desired channel at f1 (f3−f2=f2−f1). This also leads to sensitivity degradation for BS A. For example, for a middle range macro base station, assume that BS A's transmission power is 5 W (37 dBm), and after being reasonably suppressed by 75 dB by BS A's duplexer, it is still −38 dBm at a front-end of a receiver. The TX leakage signal of −38 dBm will produce IP3 products in BS A's desired channel when having an interference signal of −13 dBm from UE B. To eliminate/minimize IP3 product degradation for BS A's receiver, receiver IP3 requirement or duplexer rejection needs to be enhanced greatly. This needs high cost components and high power consumption.